You wont be able to make good solo /thermal build, if you want corruptor that functions well solo, pick /rad, /dark or possibly /kin.
A thermal corruptor is more of a team player. The thermal set is more about buffs to your fellow team mates - fire shield, thaw, forge etc. This also means that if you intend to PvP then you are probably better off choosing a different secondary.
Being a thermal corruptor doesn't mean you can't solo (just means there are better solo combinations). I have a energy/thermal corruptor which is what I would probably recommend to you. Can do good damage and benefit from knockback. You have a self heal (AoE) in warmth although its not the greatest heal, its still useful.
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i solo just fine as a fire/thermal corruptor. Admitadly it's only on villanious (or what ever the lowest setting is) but i don't die. I've even managed to take on 6 white con minions at once.
I find that the AoE heal is more than enough to keep me alive. Not tried it on any higher setting, but that's because i learnt from laying as a blaster that Blaster + boss = debt 
as for a balance build. i've managed to get pretty much all the powers from both sets.
Personlly from Fire Blast i've ignored:
Fire Ball: AoE attracts too much agro when i'm trying to heal as well
Rain of Fire: is just annoying in my experience. making enemies split up is very rarely a good thing
Inferno: It's a brilliant power, but as a helaer i need my end. and as a solo i don't have more than 3 enemies on me at any one time so it's a bit pointless
and from thermal
the second fire shild: i forget it;s name but energy powers are reasonably rare enough for me to think it wouldn't add much defence bonus
Annoyingly it seem as though i'm going to have to wait till lvl 35 to get heat exhaustion. which is annoying as i might up the difficulty setting if i could debuff enemies.
all that said... i'd agree that the thermal pool is more aimed at a team player rather than a solo as almost every power is a targeted buff.
if you have any questions about thermal or fire pools feel free to pm me about it
Can anyone recommend a good ?/thermal build for both solo and group?