you have to do the raids in a particular difficulty to have the costumes?
Yes, usually it's always hardmode. But some bosses drop nothing from Hardmode either.
45 GT drops are from the last 3 bosses in the tower, there is no difficulty setting.
50 DC drops are only from the middle door, Great Dragon kill. No difficulty setting either.
55 Teps drops only the last 2 bosses, no difficulty either.
60 Eclipse drops only the biggest bosses, but no difficulty setting so don't worry about missing out.
65 Ferris is a huge raid, here only hardmode drops costumes, and it's usually the last boss of each section (frost zone, lab zone, wild zone, otherverse)
70 Battle of Gods drops only from last boss, on hardmode.
70 Fractal raid in Dane drops nothing, regardless of difficulty.
70 Winter Palace drops only from the last 2 bosses, has to be hardmode too.
70 Secret Paths drops only from hardmode last boss, nothing else either.
I believe I covered all raids this way.
I wanted to test the mausoleum of sparks (I think it's called that) to get the costumes but I was never able to enter, you can enter and have the costumes you?
The costumes from MOS are rewarded via a currency that you get.
You get this currency once a week depending on your current rank, which depends what wave you made it to.
At the end of the layer (each mini season) you get a lot more currency, title and an aura.
You can spend that currency at an NPC for all costumes and mount skins. Unsure for F2P here, I also don't know where this guy is for Empire.
Here he is for league.
And I have one last question, on the P2P server, you know how we can have the items / costume / mounts, which we get in casettes on the F2P server?
Generally yes, F2P has to cash most of them whereas P2P gets them by gameplay. However, a lot more "new" costumes developers made are also selling them to P2P rather than put them in gameplay, which is not a good thing.
Shoot if you have more questions!