I just installed a set of Edelbrock aluminum heads on a friend's 12.5 to 1 440. I used Mr gasket 1135g steel shim head gaskets ( he provided them). Been doing some reading since and now am second guessing that choice in gaskets. Should I be worried?

They used steel shim gaskets for many years with no problems. MLR gaskets are just a different version of steel shim head gaskets. Main deal is surface smoothness of the block and heads. Steel shim and MLR gaskets don't conform much so surface RA should be 30 or better. Most likely the only issue you may have is when you pull the engine apart for service you will have a groove in the heads where the fire ring compresses. And may want to get the head surfaced to install another new gasket, but that is down the road.

probably not the best choice at 12 to 1 - I used to run a similar gasket back in the late 80's using Permatex copper coat spray on them - they usually would blow between cylinders after awhile - those gaskets do not have a fire ring so should not mar the head. the factory used steel shim gaskets and they seemed to last - it all depends on the how flat the deck is. The better choice would have been a felpro 1009.

multi layered steel is best for iron block aluminum head don't worry about it. They are better than composite unless you have a bad surface finish.

I'd never use steel shim gaskets with aluminum heads but if you have them on there you might as well run them until they cause a problem. You'll have to resurface the heads when you pull it apart. Did you verify that the head gaskets were large enough to clear the combustion chambers? Often times the OEM steel shim gaskets won't clear aftermarket combustion chambers.

Using steel head gaskets with aluminum heads and iron blocks has been consider a big No No for a very long time

I agree with Cab - the difference in expansion/contraction rates (iron/aluminum) makes a composite the better choice for longevity.
Fel-Pro 1008s or 1009s have been on my aluminum-head Mopars for years, up to 13.0:1.

I used those gaskets a few times on my builds. Stock block with 440 source heads with 10.5:1 compression. I never had an issue.

I thought the steel shims should only be used with iron block / iron heads.

Composite, copper or any non stamp steel stock type factory head gaskets

I’ve been running them with aluminum heads for 10 years. I even removed a head and put it back together with a new gasket and did not re-surface. But i’m at 11:1 compression.

Built a friends 440 with Eddy RPM heads. Steel shim gaskets at 10.0-1. It's been together and raced for years. I personally don't see why they would be any diferent than using a MLS gasket.
Doug