Edinburgh trains get the lower numbered platforms as a rule, with EK and Kilmarnock services getting the bigger numbers to minimise conflicting movements on the bridge. Just watch, though, because they do "pile" multiple trains on the same platform quite a bit in Glasgow Central, so double check the boards in case you end up going to Stranraer or somewhere equally exotic. Even the locals get caught out with that one quite a bit (as the girl going to Shotts who thought it was a new route via Kilwinning will testify...)
If, as I expect, you're coming off a Virgin or East Coast service, these also tend to use Platform 1 and 2, so a transfer should be straightforward. That said, I would not recommend a five minute transfer in any case!
The 158s tend to run as a pair on locals to Glasgow and expresses to Edinburgh. I think there's only one diagram, although I may be wrong. I don't have the diagrams to hand (although if you want to search Scot-rail.co.uk, you can), but I think that's the jist of it. It's all timed for 75mph running anyway, so a 156 could theoretically show up anywhere.
