If GUST meet the criteria I don't see any reason they should not be in the First Division. The FAI have no right to be iffy about it to be honest if they can meet the full criteria. GUST will be bringing more fans than Tralee, Carlow or whoever joins I'm sure and there is also a space there to be filled. Surely they could assemble a team to compete at worst at the bottom to middle of the division. The division is missing some decent crowd pullers next season. The last few years we have had Cork, Shels, and Derry who would all have had decent crowds (500+). These teams will be replaced by teams that might have more in the 100-150 range at a guess (and that might be being generous).
Next year the only teams that I can see pulling over 250 fans (at worst) every week to games will be Limerick, Waterford, Finn Harps, Longford and maybe Wexford Youths and Athlone. GUST could be added to that if accepted. Who knows about Cobh, they might get a bit of support back if they return.
The Division does need a Galway Utd team in some guise. I know there are two other teams to compete with in Galway but as far as support goes they would still be well a head.