Another thing to be aware of: Some 520 front extrusions have a magnetic strip at the top, full width of the table, and some do not. You can see the strip clearly in the fourth picture of the table on ebay that John sent the link for. The magnet supports the use of a direct reading scale. As far as I know, and maybe someone else on the forum is certain, the tables can be mixed and matched as long as you are not using a scale. I don't use the scale but I know some people like it. Normally, the table system would all match and either all tables would have the magnets or none would have them. That may not be practical in your case.
Based on the pics, I believe your main table does not have the magnetic strips.
I would not necessarily say the 510 tables are of no use to you. There are times when a second extension table (the kind with a base) is extremely useful to support the work-piece. You can use a non-matching table for this, the only limitation is you can't connect it to the main table with tubes (and of course it won't support the 520 fence). I regularly use a 510 table for this on my 520. You can readily convert one of your 510 floating tables to an extension table, used bases are inexpensive and you can use standard hardware to put it together.