[Paper]

The Elder God tradition (page 79 of Forbidden Arcana) specifies that for the first initiation, mages must take the Corruption metamagic (page 87 of Street Grimoire) and for the third initiation, they gain the Taint metamagic (also page 87 of SG). The question is, does this still work for PC players? Generally, PC players are forbidden from taking toxic qualities.

If the answer to this question is "yes", then a whole new question comes up. What are the toxic forms of Task and Guardian spirits? Both are on the Elder God tradition's summon list. For that matter, what about Guidance and Plant spirits?

Furthermore, what happens if a mage of the Elder God tradition takes the Dedicated Spellslinger (page 36, FA) or Hedge Witch (page 43, FA) quality? Or perhaps an Aspected Sorceror? All of these disallow the Banishing or Summoning skills, which are required to make use of the Corruption metamagic. Does this mean that the first initiation is a null for these mages; they gain no benefit? Does it mean they cannot initiate? Can they not take either of the above qualities? If so, that really is unfortunate, because an Illusion master and spreader of madness would be a fun character.

[UnLimiTeD]

I believe the strict seperation between "player" and "non-player" has been softened or dropped with FA - See Bloodmagic for the more obvious example.
I bleieve the mental workaround here is that Elder God Magic, as a Tradition, is not openly evil in the sense that 'Toxic' is - "evil" is an outdated concept based on human morality, and this tradition is played up like , f.Ex. 'Alienists' in some old D&D publication (not unlike Planar Magic) in that their Guidance is inherently either ammoral, or has aims and morals that trancend human reasoning.

As such, yes, I believe you can take those Meta-Magics, and no, you don't become a Toxic Magician just because you do.
I also believe you are free to take those metamagics if you have absolutely no use for them.
Regarding spirits - I have no idea. I believe I read somewhere that a free toxic Guardian spirit would work just like the normal one, just defending what he would normally defend against. Not of particular concern to a summoner, I believe. Gamemasters Discretion?

[Paper]

Okay, that makes sense. I definitely like the embrace of forbidden magics (almost like it's in the name, huh?) that is prevalent in FA.

The main question about the spirits is more what sort of "toxic" powers would those spirit types get? I suppose they would have to be created via house-rule. Also, my other question regarding metamagics is not so much can I take a metamagic that I cannot use, but rather am I forced to take a metamagic that I cannot use? I understand that this kind of question is a GM preference, but I'm curious about the opinions of others. Mostly so that if I make such a character, I can have a frame of reference for convincing my GM one way or the other.

I'll put an example to make it a little clearer.

Let's say I make a Magician with the Hedge Witch and Dedicated Spellslinger qualities/options with the Elder God tradition. Under Hedge Witch, "Cannot take the Conjuring Skill Group" and under Dedicated Spellslinger, "Forfeit the use of Summoning and Binding skills". I want to play this character because I think it would be interesting to have someone touched by madness who spreads that madness to others through the use of incredibly strong illusions. Now, let's say that after a few runs, I want to initiate. Under the Elder God tradition, I "must choose the corruption metamagic". However, since I do not have access to either Summoning or Banishing, I cannot make use of this metamagic. There are four options, as far as I can tell. 1) I must choose the corruption metamagic, resulting in a "wasted" initiation. 2) I cannot initiate, resulting in an inability to truly grow magically as a character. 3) I can choose a different metamagic, pending GM permission, resulting in a similar metamagic, but not corruption. 4) I can choose whatever metamagic I want.

Personally, I think that #2 would be tragic; #4 would be extremely generous; #1 would be RAW and unfortunate, but acceptable; and #3 would probably be about right. Regardless, I'm curious as to what everyone else thinks.

[UnLimiTeD]

Well, as you said, there's plenty of possible opinions.
But, if one does stack that many options with the sole aim of becoming a strong illusionist, I think #1 seems worth the price.
Aside, picking an unusable metamagic seems .... weirdly plausible for a follower of Elder Gods. 

[Paper]

Well, as you said, there's plenty of possible opinions.
But, if one does stack that many options with the sole aim of becoming a strong illusionist, I think #1 seems worth the price.
Aside, picking an unusable metamagic seems .... weirdly plausible for a follower of Elder Gods. 

You know, you aren't wrong. I like your thinking, and even though the power gamer in me cringes, the RPer in me smiles. Thank you for your input!