Ok so lets recap a little...
Wants:
TM
CQB
Lite
Able to expand
You also asked a question about the g36 and AK Beta was mentioned aswell. G36 rails can be expandable for a little bit of money. CQB is all about mobility the best way to get that is with a pistol hands down. So you want to have that versitility plus have the option for not only having 15-30 shots. So that leaves us with SMGs, Assualt Rifles with fully collapsable stocks, Assualt Rifles with Folding stocks, and stubby rifles. There are other options aswell but not sure your leaning that way or not, but here they are anyways M93r, MP9, MP7, Scorpion, M11, and dont forget the old TMP.
Going the Ak route your best bet is to try out the 74u get a feel for it. What i did to get use to a gun that i would like is well... ask people that own "CQB" guns to borrow them for a game. See how they feel ... if its comfortable or not.
Ok now its time for me not to rant... Im going to say that i more then likely am going to be useing my MP9 and g36cv for cqb. My Sig 552 for field play only... but the magic of my g36cv is i can do both hehe. Anyways back to my point i think you should think about SMGs for your cqb weapon of choice... because those can still be used in field just as good. Take JoJo's MP5SD... by all thoughts that should be a indoor gun cause of the size and type... but he still uses it outdoors. Like i said just my opinion and thoughts but for indoors you need something small to pie the cornors so your smg's will help. I am a huge fan of the p90s and UMP's but those have huge drawbacks(mags) just an idea of what to look at.
Anyways im sorry about the rant just thought i would shed some more light on the subject and maybe help.