How well is the scaIe for Irwin & Jakks figures?

The simple answer would be that they are mostly the same scale and they scale (and look) better with Irwin figures than any other DBZ product line. But the more accurate answer would be:

The Jakks Goku figures are all taller than the Irwin ones. Some are slightly taller, but the ones that share a body with Tien are much taller and larger than Irwin Goku figures.

The Jakks Vegeta figures are all a little taller than the Irwin ones.

Some particularly bad Jakks figures that utilize the same parts as Jakks Tien figures are massively too tall and large (like the Future Trunks and SS Future Trunks figures wearing gray pants and yellow boots).

If you look at some of the member collections on here, you can get a pretty good idea. My collection contains a ton of customized figures, but if there's an Irwin or Jakks figure missing from my collection, it's not because I don't have it, it's because I don't want it. For example, I don't have Irwin Yajirobe, Dende, Krillin, etc. because they are too large. I don't have a bunch of Jakks figures because they are too large. Capsulexnike's collection is good for referencing, because it contains very few custom figures. Adrenaline's collection has a ton of custom figures in it as well, but it's good for reference too. Ascat and Jibi's collections are worth looking at too. There are more... but those are the ones off the top of my head.

The Jakks DBGT figures are an entirely larger scale than Irwin and Jakks DBZ figures. They are 6 inch scale, and similar in size to SH Figuarts' overall scale.

You'll find a lot of exceptions and quirks though, because sometimes Irwin made some really weird mistakes. For example, Irwin's SS Goku in armor is much taller than any of their other Goku figures. Irwin's SS teen Gohan in blue gi is basically an NBA player and incredibly too tall thanks to arms and legs that are well sculpted, but way too long.

aidenthetoymaniac to fit in with Dragon Stars, you would almost definitely want Irwin Yajirobe, Dende, Chiaotzu, Krillin, etc. You won't be happy with any of the Irwin female characters though. They will look too small with Dragon Stars... by a lot. You will also not be happy with Irwin Frieza form 2, Nappa, or Frieza form 3 for the exact same reason.