Personality tests?

IIRC, the marriage literature found that opposites do not attract, so you could just give everyone a Big Five and try to minimize distance. As far as seating goes, I'd guess that Extraversion is the most important factor, so put all the extroverts together and the introverts together?

Divide the groups in two based on familial affiliation (they'll expect that).

Ask the following questions:

1. My radius of "personal space" is... (tiny/small/medium/large/immense)
2. I am... (short/somewhat short/average/somewhat tall/tall)

Bias x by (1) [near aisle should be "tiny"] and y by (2) [back should be "tall"]. Average groups, ignore children.

You could check out YourMorals. It's a collaboration among social psychologists who study morality and politics and has lots of tests.

Here's my current proposed list of statements for people to (strongly) agree/disagree with -- no neutrality allowed. The idea is to identify people on a farmer/forager axis, and then to cluster people according to similarity (subject to other constraints).

1. If there were cheap, tasteless, food pills that gave you all the sustenance you need, I would barely eat other food.
2. I believe it's everyone's duty to vote.
3. I regularly exercise when I should be working.
4. I think governmental oversight unnecessarily slows down the progress of science.
5. My clothes are definitely more about style than comfort.
6. I believe that travel is over-rated.

My understanding is that there are no standardized personality tests with strong evidence of validity. Using a standardized test might not have any statistical advantages over your own judgement.

ADDENDUM:

If you're just looking to avoid fights, ask people to note down their top ten bands, politicians, and books, and solve for minimum neighbor distance. Then put all the single people together and all it a day.

Is it an academic exercise, or are you planning a real wedding reception party?

I didn't see any google results for conversation compatibility, so I suspect that you could just make your own - it doesn't have to be perfect at all. Possible categories of conversations you could encourage could be things like books, music, food, relation to the new bride and groom (congrats mosb), human psychology, or travel.