Check out this related question Is a high level Elite the same difficulty as a low-level Solo, as XP suggests?
use the xp encounter guidelines laid down in DMG2 to craft an appropriate encounter and only use enemies within 1 lvl of the party (4-6 in your case)
The key reason enemies should only be within a lvl of the party is that 4e's very planned progression/math expects certain numbers for to-hit, defenses, health and damage for both the PCs and monsters. If either side is too high compared to the other they will avoid being hit and land lots of hits dealing a lot of damage to the smaller health pool of the lower level side.
As you can see for 5 lvl 5 PCs an encounter budget of 1000xp is considered standard. A lvl 5 solo would perfectly match that xp budget.
Once you've found the standard XP for your party size and lvl then look at the difficulty table to increase or decrease the xp budget accordingly
Scale it up, make the budget 1250 or 1500 and use a lvl 5 solo mixed in some other enemies to really put on the pressure.
Use the standard budget, but pick enemies that prey on the party's weaknesses
The party might be too ranged-centric, rely on too many power combos or have a specific NAD that is quite low. You can make things more difficult by creating encounters specifically to undermine their strengths and exploit their weaknesses.
You have a couple of options.
1. Switch Editions
I'm not sure which edition you're using, but Second Edition pretty much fixes this. Basically every race in the Second Edition Revised and Expanded rulebook has at least one minor special ability.
The same is true for Galaxy Guide 4: Alien Races (also a second edition book).
2. Just Do It
Just go ahead and tack on an ability or two that makes sense for the race. The races are only sort of balanced in the materials we used in our campaigns, and that wasn't much of a problem.
In general, if you give each race either a +1D attribute maximum to a specific attribute (compared to humans), or a significant bonus, you should be fine. Races with severely limiting story factors (like the Wookiee) might get both.
Mechanically, humans will lag behind a bit, although not by much. They also get the bonus of blending in anywhere, and not worrying much about speciesism.
Yes, there are female minotaurs.
Nothing in the race description specifies that the race is male-only (the way satyrs are explicitly male-only and hamadryads are explicitly female-only), ergo there must be female minotaurs.
It's just a hypothesis, but based on the people I've known who've played minotaurs, the reason there are no images of female minotaurs is that very few people play them. Also, a quick Google image search for 'female minotaur' yielded a relatively low total number of images, and most of those were... fetish-oriented... rather than heroic. There aren't a lot of minotaur images in 4e books in the first place, and the artists probably opted for the traditional depiction.