Realistic Likeness
This is a supernatural effect, thus no saving throws are allowed, and it has no duration. The bonus remains in effect until you decide to remove it.
Does not allow you to change the appearance of your equipment.
You could try to change your creature size, with a -10 on the check.
Kitsunes naturally recieve a +10 Disguise bonus to appear human (and not kitsune-y), so unless you want to mimic someone's exact appearance, this feat is unnecessary.
And finally, you should check with your GM if this feat allows you to change into anyone or just humans, as this changes the usefulness of this feat completely, like the Editor's note says:
The text of this feat states that you can mimic an “individual” which you have encountered, but it likely should specify that Kitsune have the ability to turn into humans, so GMs are encouraged to read the feat as follows: “You can precisely mimic the physical features of any human you have encountered…”
Disguise Self
This is a magical effect that works as a spell, and the save DC is based on your Charisma modifier, plus one for being a 1st level spell (thus DC 11+cha mod). Anyone who touches you, is allowed a Will save to disbelieve the effect, removing the Disguise bonus.
As a spell-like ability, it has some advantages over the actual spell. It has no verbal, somatic, or material component, nor does it require a focus.
It's important to notice that this does not change the sound your equipment does if you decide to change their appearance, so you could make your full plate look like a dress, but it will still clank when you walk on it (which also allows a Will save).
So, unless your Charisma is really high, here and then someone will disbelieve your disguises, because the save DC won't scale with your character class levels.
It also has a duration of 10 minutes per caster level (which is your HD). So at most, it will last for a few hours.
Using Both
Yes, those two abilities can help each other, Realistic Likeness grants a circunstance bonus, while Disguise Self grants an untyped bonus. So if you happen to be using both abilities at once, you could recieve up to +20 on your disguise check as long as both are to appear exactly like a specific individual.
Again, these bonus ALSO should stack with your Change Shape's ability bonus, as it is a racial bonus, as long as the form chosen is that of a human. Which is why you should check with your GM regarding that Editor's Note.
Detect Magic
People casting Detect Magic are your bane. All three abilities are magical in nature and will ping when someone casts Detect Magic around you. Though Disguise Self is the only one they can identify with a Spellcraft check, the others will at least show up as a magical effect on your character, which will raise suspicion.
Best Answer
I don't think that there's going to be an official answer to this by the DnD 5e developers (e.g. Sage Advice), because the Acquisitions Incorporated book was written by the Penny Arcade team, and was simply published by WotC.
My interpretation of it, however, is as follows:
Your spell affects an object, and covers the entire object. That object must have dimensions of no more than one foot in any direction on each of its sides. So, for a cone, you can't have a diameter of more than one foot on the base, and the sloped sides can't be more than one foot from the point of the cone to the base, measured along the sides of the cone.
This means that, depending on the shape of the object in question, it can be possible to cast this on an object that is bigger than 1 foot across - for instance, you could cast it on an icosadecahedron (d20) whose triangular faces each have a length of 1 foot.