Knowing what era you model. would help in giving you advice. Later era diesels, you can pretty much run almost anything, anywhere and consisted with just about anything else. Now days with pooled power, run throughs, power by the hour trading and leasing, power often ends up FAR from "Home Rails"
In the Steam era, shared power was much less common, but did happen, such as the DM&IR Yellowstones working on the D&RGW during the winters when the DM&IR was shut down due to the Great Lakes being frozen over.
The farther back in the Steam era you go, the less power you see on foreign rails.
There are some advantages to "Focusing" your collection to a few roadnames. By focusing on a few roads, you are subject to fewer Impulse purchases, and that can save you quite a bit of money.
It can be very interesting to learn more about the roads that you have chosen to model.
If you choose to model roads that actually interchanged with each other, it can add a lot of believability to your layout.
For many operators the "Toy Train" style is what they enjoy, and there is nothing wrong with that, I got my own start in 3R O Gauge with "Traditional" size for the Christmas Tree. It didn't take long for the "O Gauge Virus" to bite me, I then discovered Lionel's "LionMaster" line, and at that point, I pretty much abandoned the HO side of the Hobby, after more than 35 years of HO. Then Lionel came out with the Scale Union Pacific FEF-3s, and I was hooked on scale size equipment, though there are often "Traditional" size products that I like as well.
You know yourself better than anyone else does, so it would be a good idea to decide just how "Focused" would make you Happiest, whether it is Roadnames, Era, Size or type of scenery you choose.
That is one of the best aspects of this hobby is there are so many ways to Enjoy it, You make your own rules.
Doug