how much of a difference should there be between what ur open raising with and what you isolate raising with?

Not much.

To be more precise, about 1 more big blind.

is there ever an argument to have a tighter isolating range then opening?

Depends on who you are playing, if I am in position against a loose/passive fish with 100BB+ stacks my iso'ing range is probably wider than my opening range.

Open-limping isn't as common at some of the stakes that I play. However, when it does happen, I want to encourage it to keep happening, so I probably don't iso as much as some other people. If you're iso-ing too much, you're asking to get limp-re-raised or 3-bet marginally by competent players who catch onto what you're doing. There are a lot of times that I'd open Axo or some other unsuited connectors in the CO or on the button, but I'll fold them when faced with a limper. I also don't mind playing limp-along with some speculative hands on occasion to encourage more players (the blinds) to come along...if someone does raise, you get to see what the field does and have position in the pot...whereas if you'd raised and are 3-bet, you're going to have to fold frequently.

It depends.

If the limper is loose preflop but folds frequently on the flop, your isoing range can be very very wide.
If the limper is loose preflop and calls too much after the flop, your isolating range needs to be a little narrower.
If you have players who frequently call raises too often preflop to act after you, this should narrow your isolating range as well.
If you have players who frequently 3-bet behind you, this should narrow your isolating range.
If the player is very shortstacked, your range should be narrower.

Agree with all but bolded.
Depends on how they got very short, I guess.

As stacks get shorter, Ax and broadway hands, ie KT, QJ, JT, KJ, etc go up in value while hands like 76s, 64s, and the lower pairs go down in value.