- About runes there is information in Runes section, offering other options, same about boots in item section to escape CDR overcap.
-About HP per level seals, it is what I find best for myself, flat armor seals I find better early game in lane phase, in mid or late game I preffer extra hp, as in team fights You will get hit with all kind of damage. But I agree that You can go MR+Armor aswell, If You find that it suits You better.
-About Frozen heart I agree, but if none of tanks buys it, I still go for it every game then.

70% cooldown reduction is really just overkill, switch out your glyphs for magic resist per levels or your boots for boots of mobility. the cap is 40%, its ok to go a little above that but 70 is too much. Also, health per level seals? what? use flat armor seals and i would use GP10 quints as well. Frozen heart doesn't fit soraka at all. Better if your tank buys it.

you should turn comment to vote on. But i'm a pretty avid Soraka player and I definitely approve this guide. Quality info all around

Tristana just has terrible synergy with Soraka overall. This is due to her being strong early game AND late game but weak mid game, according to TSM's Chaox and other prominent AD carry players. She cannot utilize the mana sustain from Soraka well. Before level 6, her only source of burst damage is also her gap closer. And Soraka doesn't bring enough damage or CC to complement her when she wants to play aggressive. The only good thing about a Trist/Soraka lane is you can babysit her to late game, where she is very strong. Other than that, you could say it's not a bad combo... but it's not a good one either.

About runes i agree, will put them in.
About Tristana- can't agree that she is bad. She, perhaps, does not have as good synergy as other champions, but it all depends from player. If player choses the correct moment when to move in for a kill, there is no problem. Playing in really high elo maybe this problem is becoming more actual about setup but otherwise not.

Why no Greater Quintessence of Gold under alternative runes?

Also, about Tristana. She is the only carry in the game who is bad with Soraka. She has no poke, isn't particularly mana hungry, and Soraka isn't aggressive enough to complement her when she forces a fight.