As above.
An Elysian model with a Lasgun can easily be used as a Guardsman / Veteran in a standard Astra Militarum army. I can't remember if they have access to ususual equipment, like a "Lascutter" or something, but you just wouldn't be able to use that model as a WYSIWYG anything... except maybe a meltabomb.
They have Drop Sentinels, that you probably couldn't use with the Deep Strike rules, but you could still use them as regular sentinels.
And that's only if FW is disallowed. Not everyone disallows FW, just some. So if FW is allowed, you could run them as battle brother allies to a standard Guard list.