Yes I did it with only 5 total Pokemon. This includes HMs. Pokemon are sorted in order of usefulness. Sing with a bulky pokemon is op, especially with Generation 1 sleep mechanics. Beedrill underrated in Gen 1, cleans out Ericka, Sabrina, Koga, Lorelei, and most of the team rocket encounters. Literally the only issue is that sometimes he crits, which undoes your +6 attack. Charizard is lowkey cheeks, but dig being 100 bp is really broken. Seaking is there because I needed to surf, and a water type makes fire old dude, Giovani, Bruno and Lance free. Pidgeot is theoretically not needed, but I like being able to fly and it helps get rid of trash encounters for the rest of this series. I think I only used her to cheese with Sand-Attack.
And no, beating Red with only 5 Pokemon ever used with Hardcore Nuzlocke rules is not hard. I think you could theoretically get down to 3, which someone has probably done before. Generation 1 is buns easy. I have way too many rules, but more or less standard hardcore nuzlocke (first pokemon encountered on every route unless a repeat, level caps, no items, set mode, can only use same amount of pokemon as gym leader, etc.). In order to help my sanity, when i catch a pokemon, even if I never use it, it'll count for dupe clause on that specific game. Oh yeah, and no double team unless a Pokemon naturally learns it.
If it wasn't clear, every Pokemon that I use (i define 'use' as a) sent out in battle b) taught any move at all c) given/taken an item from) in one game, i can not use ever again. The only exceptions are that previously used pokemon can be hm slaves (they cannot under any circumstance be sent out in battle or given items like xp share to prevent other pokemon from over leveling), and I'm allowed to catch any pokemon i want for specific items, i just have to release them immediately after (like catching magnemite for magnets).
I'm not expecting Blue to really be any more difficult, I either get to start with a Bulbasaur or a Squirtle, which are both 1000x better than Charizard.