There's no need to house rule this — you just need to start enforcing its restrictions. Allow me to draw your attention to its area of effect:
4 square″ + 1″ square/level
(Aside, this is taken from PHB page 95, since the Illusionist version of the spell has different range and area of effect.)
These are table inches, which we could convert to feet/yards, but the following will be simpler if we just compare table inches directly with fireball's numbers.
A fireball has a radius of 2″. Let us ask then, “what level would an illusionist have to be to cast a phantasmal force large enough to contain a 2″ radius sphere?” Let's make a first-order approximation using the minimum number of 1″ squares necessary to create an area in which the image of the sphere could appear. The area of effect of fireball is three-dimensional, but the area of effect of phantasmal force is two-dimensional, which presents an odd issue. However, phantasmal force is described as being able to create images “within the boundaries”, so let's be as generous as the spell seems to be intended, and say that the vertical space it can create the illusion within is “good enough” for our purposes and we just need the footprint of the illusory fireball to fit inside the phantasmal force's area of effect.
We'll assume as a first-order approximation that each 1 square″ of area of effect can't be divided, so they have to be tiled as whole square table inches. Inscribing a 4-unit sphere's footprint (a circle) inside a pixelized shape requires a 4×4 square of squares, or 16 tiled squares.
So, as a first approximation, an illusionist needs to have 16 square table-inches of area of effect to play with to be able to make a phantasm of a full fireball. That requires being level 12 to pull off.
But maybe you think tiled squares aren't generous enough. Let's allow the table-inch squares of area of effect of the phantasmal force to be subdivided infinitesimally so that the area can be a cylinder that just barely contains the footprint of an illusory fireball and see where that gets us. (Some DMs might reject this move since the area of effect is given in squares and they may require a straight-edged area of effect, but some others might allow it with the reasoning that it's fine as long as the final area matches regardless of shape.)
A 2-unit-radius sphere has an area through its widest point of just over 12½ units, rounding up to 13 square″ required. So even being super-generous, it would still take an illusionist of level 9 to use phantasmal force to create an illusory fireball.
Clearly, creating illusions of fireballs with phantasmal force isn't something that your players should be doing at first level, or even 5th level when their magic-user friends are starting to throw real fireballs.
Conclusion: You don't need to house rule, just give the players a reality check
So, there you have it: you don't need to house rule this at all, you just need to pay proper attention to the relatively small area of effect, and break the news to your players that you've been accidentally letting them create a 9th-level illusionist's phantasmal forces before they were actually 9th level, and won't be doing that any longer.
If they are 9th- or 12th-level, then this isn't overpowered at all, because they have access to equally (or more) powerful effects already and cleverly using their resources like this should be letting them paste low-HD enemies by the truckload. So again, there simply isn't a problem if the area of effect is enforced.
Addendum: Don't overlook the audio limitation either
The above is all completely ignoring the impact of the stipulation that a phantasmal force is silent. Is a fireball silent? Not at all. An illusory fireball with no sound effects is not very convincing, and unlikely to fool even stupid opponents, even stupid opponents who have already been hit by a real fireball. In fact, having just experience a real one with all the sound and fury of real fire, they'd be more likely to notice that a silent one is not quite right.
This is a very tricky question; but a very good one. The primary deciding factor is not rules, be they written or interpreted, but the Dungeon Master's ruling in question.
Since Phantasmal Force creates an illusion so vividly rooted into the afflicted target's mind that the target's thought process rationalizes any interactions with the illusion (an illusion ONLY the afflicted creature can see) to be perceived as real, and an illusion made to look like a creature can deal damage in the form of 'an attack' against the afflicted target, I would rule that yes; Phantasmal Force creates such a realistic illusion as to create opportunities for the 5E Advantage system (granting the rogue sneak attack as below).
"...you can deal an extra 1d6 damage to one creature you hit if you have advantage on the attack roll..."
Keep in mind that I am saying the illusion would grant Advantage (one of the factors a rogue needs to use sneak attack) , not that it counts as an ally creature within 5ft of the target (see below); because the creature is an illusion only perceived by the target and is not physically there for all other intents and purposes.
"...you don't need Advantage on the attack roll if another enemy of the target is within 5ft of it..."
This same reasoning can be said about flanking. If flanking states that the enemy in question must have a physical threat on opposite sides of it, I would render the target not flanked, but certainly distracted or concerned enough with its illusory threat to be unable to put up a proper defense against a REAL enemy coming up behind (or beside) it.
Other Dungeon Master's might interpret this differently and disagree with me. So in this case, point out the entire Phantasmal Force entry to your DM and let he or she make a ruling.
Yes. It can move wherever the target goes (assuming the thing created can move).
Ultimately, it is the DM's decision however I think the evidence definitely supports it being able to move with the target since it is in their head anyway.
Jeremy Crawford's tweet (emphasis mine) indicates that the size of the illusion/phantasm is limited to a 10ft cube.
Note he doesn't say "stays within the 10-foot cube"...
Couple that with the fact that you target the creature not an area or point in space for this spell to seed the mind of the target with the phantasm. It does not anywhere indicate that it can't move out of the 10ft cube it starts in and since everything occurs within the victim's head there would be no reason that it couldn't if it were an imagined creature.
This means that when you cast you target a creature within 60ft with the idea of a creature or object no larger than a 10ft cube. After that point for the duration (assuming you chose something that is ambulatory such as a lion) it will follow and attack the subject of the spell.
Mike Mearls' tweet also indicates that such a phantasm need not roll to attack; it just automatically does damage. I would expect the text of the spell to indicate as much if it were required.