So last weekend, I snapped about 7-8” off the end of one of my St Croix rods. Filed through warranty and will be replacing it. That said, I want to still use this rod. I know if I was building a rod and wanted it shorter, I’m supposed to take off the butt end because the action might be compromised if taken from the tip end (AFAIK). But, can I just put a tip top on this rod that matches the last running guide in size? Or, is this just a junk rod now?

It's junk now. That's way to much to get any kind of action out of the tip. 2 inches makes a big difference.

Take a guide off the broken tip and tape it securely the what's left and try it, you don't have to guess or spend a penny, I've used guides as tup tops before with no issues.

You can repair it and probably won't notice any difference. I've done several.     https://www.rodbuilding.org/library/repair-oquinn.html

Alex has it right.  While many don't believe it, the process works well.  I also have done many, and while the sleeve sort of looks screwy, the action and power will be the same as before.  Because of the added weight way out towards the tip, the recovery speed from deflection will be slowed a little.  But probably not enough to really tell.  I broke one of my rods I had built, and had CCS numbers before and after the failure/fix, and they were unchanged.

 

Simply putting a new tiptop on will result in a very poor solution as the power will go up and the action will slow down.  And the rod will feel nothing near what it was before breaking.

Glue on a new tip, fish it and see how you like it. The most it will cost is the price of the new tip. Not much risk involved. I broke the tip off the same rod twice losing 3-4 inches. I can’t tell the difference. 

Finally got around to looking at it. The tip broke just past the next to last running guide (so the broken section has one running guide and the tip top). I’m thinking I’m going to epoxy a small bit on the end of the rod so as to not get line fray. Then, I’m gonna go fish it. At worst, I can cut it down and use it for a spoiling setup. Maybe one of the more expensive ones albeit! 

Before you do that, take another look at    https://www.rodbuilding.org/library/repair-oquinn.html