Before I answer I am an amateur that has not done this for over 30 years. The stage sound gear I read about today is fantastic by comparison.
When recorded music is played through the hi-fi that music has been "produced." It will have well controlled maximum levels, possibly some compression, some limiting (heavy compression) and the sound level will be the highest the CD/MP3/Vinyl/radio broadcast can support. That music will sound quite loud and most records will be about the same volume give or take a bit. The noise floor (in the gaps in music) will be well under control.
Not so in pub gigs. When you play live with perhaps a small mixing desk, the mixed sound is unlikely to be as well-conditioned as recorded music. Consequently, your pub's hi-fi will need to reserve more headroom to accommodate the variation in sound levels the performers will produce. If you have multiple mic's sound from other instruments will bleed into other mic feeds, which can make controlling the sound mix harder.
My preference would be to use the pub's hi-fi for the vocals first of all and work out how loud you can go before feedback / howl or ringing. If the the other instruments are loud enough without further amplification you are ready to go. Use a directional mic for the vocals (e.g. cardoid) as it will help reduce the risk of feedback.
In my opinion there is nothing worse that inaudible or muffled vocals. The rest of the band need to adjust their volume relative to the vocals.
If you have an instrument that needs a little help to be heard you might add that into the mix feeding the pub's hi-fi. The more instruments you add the greater the risk the sound will deteriorate - become muddy, muffled or just rough. I would leave the drums out. A drummer I knew many many years ago used to out-drum an 800W sound system quite comfortably. A drum-kit can suck too much power from the hi-fi's power supply, which takes time to recover if it is not meaty enough.
If you are playing at a range of venues, you will have to change the sound mix, so keeping a permanent record of what worked and what didn't could help setting up and sound checks.
One problem might be the absence of fold-back or monitors. Apart from the vocals, the performer has the volume control for his/her instrument. If a performer hears their sound relative to the rest of the band and thinks it is too quiet, that performer will want to up the volume. A gradual race to 11 might happen between band members, especially toward the end of the set. I guess the band needs to practice playing without fold-back / monitors / wedges until each performer is comfortable and confident they can be heard at the right level to make a good performance.