Reading the diplomatic landscape to find the best alliances is an underrated skill. However, I'm not so sure that Muscovy/Russia is a great choice. They're likely to draw you into wars against Kazan etc and won't be that easy to call offensively due to distance and their weak early economy. They'd be valuable against the PLC, but that's not a high-priority direction to expand anyway. Mamluks are probably a better pick for a friend (usually possible thanks to having the Ottomans as a common enemy) since they force the Ottos into a two-front land war and have a decent navy. Bohemia and either France or Spain are reasonable picks too.
Generally speaking, I divide potential allies into three different categories depending on long-term plans:
1. Longterm/permanent allies - countries in a strong position to help out, but where you have no intention to expand. These are ones where you should always make sure are as strong as possible by helping them expand, giving them lots of land in peace treaties and actively supporting them militarily.
2. Medium-term allies - major countries on the opposite side of your main enemies, but that you plan to eventually betray once those are down. They work both as a deterrent and weapon but are allowed/encouraged to take the brunt of the damage during conflicts.
3. Short-term allies - countries that really aren't interesting for more than being called into a single war with a promise of land that you (typically) intend to honour in order to give some extra armies or opening extra fronts. Since you probably will break the alliance after the war ends, going over diplo relations for this isn't a big deal. Note that the 'threatened' status towards your target almost guarantees that they won't join you in the war so check both that and make sure they actually want any land from your target before suggesting such an alliance.
Generally speaking, I divide potential allies into three different categories depending on long-term plans:
1. Longterm/permanent allies - countries in a strong position to help out, but where you have no intention to expand. These are ones where you should always make sure are as strong as possible by helping them expand, giving them lots of land in peace treaties and actively supporting them militarily.
2. Medium-term allies - major countries on the opposite side of your main enemies, but that you plan to eventually betray once those are down. They work both as a deterrent and weapon but are allowed/encouraged to take the brunt of the damage during conflicts.
3. Short-term allies - countries that really aren't interesting for more than being called into a single war with a promise of land that you (typically) intend to honour in order to give some extra armies or opening extra fronts. Since you probably will break the alliance after the war ends, going over diplo relations for this isn't a big deal. Note that the 'threatened' status towards your target almost guarantees that they won't join you in the war so check both that and make sure they actually want any land from your target before suggesting such an alliance.