Do keep in mind that Marimo Moss Balls are actually algae. At least the real ones are. Any of the ones sold in pet stores would be just fine as far as size goes. Only under good care and having it for MANY years would it get to big for that tank. I have 3 in a 5 gallon tank. They are not the micro ones you often find online (which are also just fine, but I am just trying to say that you probably won't get one that is too big).

As far as Java moss goes, people often have trouble finding it depending on where they are located. I think that I have Christmas moss, which is often sold at Petco as "Aqua Turf". I am not sure how well it would do with being moved around (since you can't cycle anything under 5 gallons). Not even sure it has roots...
Not sure about adding to many at once, but they do need nutrients, so make sure you don't have too many in a tank if you are not adding fertilizer (but no fake CO2! This is bad for moss. Real CO2 is okay, but that is a whole different level). For such a small tank (for plants, not fish), you are probably best to just stick with moss balls and see how you like them before trying more advanced stuff (as advanced as java moss is to those more experienced people... lol).

 

I have some Java moss still in the petstore bag.

That stuff feels like soft razor-wire to me. I'm not sure I'd ever feel safe putting it in a tank with long-fin fish.

 

Java fern shouldnt feel like razor wire do you have pictures...? What variety java fern is it?

 

What variety java fern is it?

Java Moss. I had a problem with fin-splitting on one of my fish. He even split his dorsal on a silk plant while in QT for a split anal...I'll never figure out how that happened.

Anyway, I got some Java Moss for my first live plant. As soon as I took it out of the bag, I knew I would never get it near any of my fish. Might be fine for fry or shrimp to hide in, but not for rambunctious Betta, IMO.

 

All mosses are fine to add to your fishtank, its one of the softest plants around. It does take time to get it to grow but once it does its a great plant and easy to care for. I have java moss and chrismas moss in my tanks and the xmas ones grows faster than my javamoss. I use seachem flourish and excel with both my tanks and strangely enough since Ive started squirting the excel near my mosses they started growing mad. Now I use a needle/injection to add my nutrients since its easier to get the 1ml dosage right for my 5g 2 times a week.

 

That is odd my veil tail male sleeps on his terracotta pot which is covered in javamoss and he has never damaged his finnage from that.

 

You didn't say he sleeps IN his moss.

I don't hear horror stories about Java Moss. It's just that I have a tailsplitting energetic fireball of a Betta, plus his brother who is only slightly less rambunctious. I should already have gotten rid of their silk plants, but I haven't yet found replacement shade that they can also hang out and rest in. Probably Java or Amazon Fern.

But the day I took the Java Moss out of the bag and stuck my hand inside it, I knew I'd never put it near fast energetic soft-finned fish.

Don't know anything about Christmas moss. I'll evaluate that whenever I might see it.