This sounds like an opportunity to introduce a more Nethack-ish feel. What we can do is:
- Use the random polyp tables in advance to make a secret list of 20 random polyp forms.
- Number the list from one to 20.
- Take the six parasitic attacks from the first polyp table and assign them in order to the numbers 11 through 16.
- Make numbers 1 and 20 vampires.
Never describe a polyp as an "Ego Parasite" or whatever its feature is. Describe how it looks. You can come up with short nicknames that incorporate the appearance, for when the players encounter that variety again. "You see five of those Bony Barbed Balls you encountered before."
If you roll an ethereal encounter above ground, roll 1d6+1 to get the variety. All of these polyps will be harmless, so if players start out using Rope Trick a lot on wilderness or urban adventures, they will learn all six varieties and assume the polyps are benign.
In lonely ruins at night or at a high altitude, roll 2d6 instead. There's a few new varieties of polyps to shake things up, and a rare chance of getting Ego or Superego Parasites, which will shake the players up a bit when they first encounter them.
If the encounter is below ground, roll 2d6 if the level is 1 to 12, or 3d6 if the level is 13 through 18. Thus, deeper levels have a much wider variety, including new parasite types. Plus, parasites are more common.
If the encounter occurs on level 19+ or in outer space (assuming that's possible,) roll a straight d20. There's a pretty decent chance of a parasite, plus now the vampires may show up.