[Radek]

1. Yes, your reputation will go down in both guilds, even now. That's because Telvanni and Mages Guild don't like each other. But it does not matter too much. Doing quests will restore your reputation faster than you will lose it by doing quests in the other guild.
I have played both guilds several times. In all cases I was able to become an Archmage of the Mages Guild and the Archmagister of Telvanni.

It's the same as with other guilds. There is a small group of guilds, which members do not object against being a member of another member of the group. All other guilds like only themselves and they will object against your becoming a member of any other guild.
For example, the temple. It likes only itself. You will lose reputation at the temple if you join any other guild and if you do any quest for the other guild. The Telvanni is the same story. It also likes only itself.

2. Recipes:
Fortify SPD = Moon Sugar + Kagouti Hide or Shalk Resin
Restore SPD = Raw Ebony + Resin