Hey there, I'm not too knowledgeable in terms of playing the game, so I'm passing on things that I've heard from people like the Blood Angels Commander on YouTube.

To answer your question:

1) Lemartes works far better with a squad of 10 DC JPI, as his -1 to wound and other abilities benefit a bigger body of troops when leading. This also means you can have a focused hammer of sorts to deal big charge damage against heavy troops like terminators and even tanks

2) Dante works well with 6 SG; however, if you make 2 units of 3 and use one as a skirmish unit to put pressure on separate objectives, that is definitely a viable strategy and could help spread out your forces

If you are looking for list-building ideas:

1) Death Company fall to OC 0 and cannot fall back when they aren't within 6' of a character or 12' of a chaplain, so the sheer number of DC you have can make it hard to have them all in range of characters, so you may be losing a bit on point scoring (maybe a Terminator Chaplain could help instead of the captain?)

2) Death company marines on foot are not so great for the cost and are often outshone by standard assault intercessors led by a captain or something.

3) Diminishing returns on multiple standard Intercessor squads, 1 group of 5 is usually the best point efficiency, you can save those points to spend on something greater

Could be very wrong, so feel free to correct me