Do not IV Seroquel, it's an antipsychotic and not recreational in the slightest, and you can have a really unpleasant reaction called acute dystonia where your neck, face and eye muscles go into spasm and you need emergency treatment; I would imagine an IV dose would have a higher incidence of that.
Speedballs are dangerous because the upper can mask the downer and lead to an overdose when it wears off.
We're not here to help you get high; if you have specific harm reduction queries about particular drugs do come back and ask, but we can't advise you what to take, sorry.
Closed, PM me with any queries.