Inserts and adapters are two different things. Inserts, either aluminum or brass, glue into the shaft and are female threaded for accepting a screw-in broadhead. Adapters, either aluminum or steel, are for gluing a glue-on point onto so you can use them with carbon or aluminum arrows, they are male threaded on one end and tapered on the other. There is someone making steel glue in adapters, a combination of the two, but in reality, they make it difficult to change point weight for tuning purposes.
You could go either way, 100 gr. brass insert plus your 125gr. broadhead, or get new glue on 125gr. broadheads and glue them onto a 100gr. steel adapter. Since you're shooting full length and you want to add weight to the point end, thereby weakening the spine, you've got enough extra arrow to cut them back to stiffen them back up. Try just adding the brass insert without cutting and see how they fly, if obviously weak, then start trimming the shaft back a little at a time until shooting good again. Take your time, you don't want to rush and make them too short!
Hope this helps!