Yes there are mod servers(no DL req just server setup) for 8v8 there are even some with 32(16v16). As for the toons it just randoms it, you'll usualy have 3 hunters 1 jockey, spiter, boomer, charger. There has even been games with all 8 coach which is kinda funny when they get into their dialog at the begining of the rounds.
I play on the WTF 8v8 all the time and since its a laggy pice of shit I thought it would be bad ass if TD made their server(awesome power beyond all belief) 8v8 or even 10v10.