most serra species needs to be kept solo including rhom, unless you have that much cash to burn then you can "try" to put more in.
don't believe aquascape has any big ones in now, only very small ones at 3"-4" but at that size it's very hard to ID it rather it is really a blue diamond or not. (not that I don't trust pedro but just it's just hard in reality)
personally I wouldn't get anything smaller than 6" especially you just start to get into piranha. if you have the money best is to get one at 10" plus as they grow slow and not as entertain as pbass so if you start with a small on you might get bored quickly.
here's an old video of my blue diamond
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and some pix. it's 8" thinking about selling it so if you are interested pm me.