Sometimes a small group can evade notice in places where a larger group would simply be meat for the grinder. If they don't have the brute force to pull this off (which at that level, they won't), they're going to need stealth and guile to even survive, let alone succeed.
The FC2 mentions Zariel as the source of some of Bel's power, and of the fireballs that feature so prominently on Avernus.
Dis is pretty well detailed in FC2; the whole "cannot be navigated by nondevils" makes the proposed adventure questionable in its sanity (beyond sending low/mid-level PCs to Hell).
as far as i kno 3 out of the 5 will have adjustments (nothing crazy like a rakshasa's plus 7)
Level adjustments are almost always a Bad Idea for PCs, especially at low levels (unless you allow LA buyoff).
ive boon looking through for demons/devils to fight since they will be going through through the first layer which is a eternal battlefield for demons and devils and they will have to travel through the city without drawing attention to themselves. There will be strong fights, they may run into something easy, and there will be things they might have to run from
Given the organization of Hell's inhabitants, a simple imp could escape the party and provide more powerful devils with the party's location. In order to pass to Dis, the party will have to pass in front of Tiamat's lair
, something even high-level characters might have difficulty doing. They will have difficulty running from something like a squad of barbazu, an erinyes, bezekiras, or imps. If they encounter a cornugon or pit fiend, there is no way for them to avoid a TPK, short of swearing a blood pact that they will serve the devils and return to the mortal plane to advance the agenda of Baator.
Unless the party is already Lawful Evil, this adventure should probably end within a half hour of them reaching Avernus.
A more suitable location for relatively low-level planar adventurers would be Outland or the Astral Plane (watch out for those astral dreadnaughts, though!). The Astral Plane can take characters just about anywhere, so you can move them to the Lower Planes when they're ready. By following the Styx, they can move from one plane to another, if they're brave enough.