IIRC Garrus didn't have to be recruited in ME1. If not the logical assumption would be that he would have died in the attack on the Citadel. I'm not sure but I think you can play the alien card and you might not "have" to recruit Tali.
This didn't keep either of them from showing up in ME2 though.
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For example:
-- If you need a Quarian and Tali dies there's always Kal Reager [sp?] or a generic Quarian Marine if Kal also died. Maybe even Veetor??
-- You could always recruit the Krogan female [that had the Shep breeding request] if Grunt dies
-- Someone could stand in for Mordin [one of the other Genophage developers]
-- Jack could be replaced by "random Biotics expert"
-- Samara / Morinth are quite possibly replaced by Liara
-- Zaeed is just a hired gun ... any replacement will do
-- Miranda has a built-in twin sister [esp. if there is a few years game gap]
-- Jacob could likely be replaced by "generic Cerberus OP"
-- An archived copy of Legion could be redownloaded to another mobile platform
-- Garrus [token Turien / CSec] practically didn't exist in my game. I should have told him to forget the forward batteries and come play poker with the Engineering staff more often. Maybe he'll have more to say to my FemShep that I'm currently playing through
-- Thane ... A Salurian may be able to replace him as far as stealth skills and intel go. Heck Liara could also help in the intel department
Now on the other hand if the characters [Tali and Garrus] from ME1 that made it to ME2 are just "throw away" characters then I'm all for gathering torches and pitchforks

I don't mind losing these characters during a playthrough -- Tali as a Quarian rep coordinating war efforts with other species [or an Admiral?] or putting Garrus on a new council after the "Reapers" take care of the old council would be perfectly fine by me [at least it's MY decision]. As long as the Epilogue is very long, detailed, thorough then I'd be happy with losing these characters as long as I felt that I was in control.