I'm having a non-traditional processional.

We also had a non-traditional processional (and even shorter than yours). We had no wedding party at all, so we had my husband's mother escorted by his older brother, followed by my mother escorted by my older brother (who was also officiating), followed by my dad and I. The DJ did a great job of timing the music and it worked out perfectly. My husband did not want to walk, so he started at the alter and it worked out perfectly having my brother/officiant escorting my mom.

So you should do whatever feels right to you. Don't change your plans based on what you think you should do or what anyone else does. If you want people to walk in pairs, you shouldn't change it just to try to make it longer. And if you want your officiant to escort someone, go ahead and do that.

Best of luck!

We had a small non-traditional wedding party also. For starters, our sides were not even. The groom had two men standing with him, and I had three on my side (two bridesmaids and a bridesman). Since the sides weren’t even, we had the wedding party sides walk in together in a single file line- the groom walked in with his groomsmen following behind him. And my bridesmaids and “man of honor” walked in after them. My brother escorted my mom down the aisle, and my father escorted me. Our Officiants were already at the altar waiting, so they were not part of the processional.
It was short and sweet just the way we wanted it (because to be honest, as a wedding guest no one likes sitting through long drawn-out processionals). And it actually worked out great with the music! Rather than being one of those weddings where people choose separate songs for each group that you have to orchestrate, either by finding shortened versions of songs, having them each fade out, or sitting through music while nothing is happening (all of which oftentime results in the wrong songs getting played or long awkward silences between songs), we were able to use one song for everybody. Personally, I don’t think it looks more formal to have people walking in pairs. I find it looks stuffy and kind of awkward. Most the weddings I’ve been to as of late have had everyone walk in separately (with like 5-6 feet between them if it’s a long aisle). This looks and feels a lot more natural in my opinion, rather than forcing strangers to awkwardly link arms down an aisle. Having your FFIL walk in first with his wife would be fine. Another option would be to have him walk alone, and your FMIL escort the groom down the aisle.