+Atk -Hp Adult Tiki acceptable?

Not great, wait for -Spd. You'll get a lot of A!Tiki, she is quite common.

its not the WORST but its not that good either
A!Tiki is fairly common so it wouldnt be that hard getting a +Atk-Spd one (Which is the best IMO)
but if you want to use tiki now, go for it

Nope, best are -spd +what you want, +atk, +res( for res lb++) or +def.

since she can appear as 3 and 4 stars i would say wait for the optimal IVs dont be hasty