I have a late 90's bike with 600 Ultegra that's always been a little too big (even after adjusting the seat, stem, etc.). Been thinking of buying a new lower cost steel frame like a Surly or Soma. Would appreciate any guidance regarding what I need to look for in the bottom bracket to ensure that my 600 ultegra will migrate to the new bike without serious issues? I'd like to re-purpose as much of the original bike as possible.
This is one of several bikes, and I'd just like to get a steel bike back in the stable.
I've seen references to a "square" bottom bracket, but don't know what that means.
The parts should mainly move over without too much trouble. The bottom bracket won't give you any issues. It is a standard (or what used to be standard) square taper bottom bracket. Both bikes are english threaded so that won't be an issue.
Here is where you may have issues. The front derailleur might be the wrong diameter size so you may need a new front derailleur. The seatpost diameter may not be the right size for your bike. The brakes may have the incorrect reach for your new bike. The headset is one inch threadless and 1 and 1/8 is more common today.
Your idea should work but you may want a bike shop to install the headset and the bottom bracket and you may end having to buy a new headset, front derailleur, seatpost, and possibly new brakes.
What kind of bike frame are you wanting? You mention Surly and Soma, but would you go with a road bike such as Surly Pacer, or their all-rounder Cross Check? Makes a difference in whether you may re-use your brakes. Cross Check, or Soma Double Cross, are great frame sets because they can do so many things well.
Just to flesh out my previous post, if your main goals are steel, fit and reusing original components, it would be easier to find a vintage steel bike on your local CL probably. Any components you don't use you can hold in reserve just in case or sell them. Or, move those components over to the Allez and sell it whole. Could turn out to be more of a swap than a purchase. Then you get what you want and it is paid for.