The planet has an average of 8200% tidal force that the moon has on Earth, so organisms would use tidal energy for metabolism.
No problem with that. Proximally, they'd be using geochemical energy, with tidal flexing providing the heat source to produce chemical gradients suitable for chemosynthesis. The ultimate energy source may be different, but the biology would be essentially identical to what already happens on Earth with chemoautotrophs.
Because of the nearby brown dwarfs, some organisms would convert magnetic energy to energy they could use.
That is infeasible. The only way organisms could conceivably exploit the magnetic field of a host brown dwarf is by growing electrical conductors of some kind in which to induce electric field gradients as their world orbits through the host star's magnetic fields. At the range of field strengths that are actually physically realizable in such a system, "magnetovores" would need to be tens, or even hundreds of kilometers long in order to develop large enough voltage gradients at one end or the other to be useful for any kind of biochemical usage.
The brown dwarf's magnetic field could indirectly provide another source of biological energy, though: by trapping charged particles to produce radiation belts, it could result in periodically irradiating the planet's upper atmosphere, producing energy-rich molecules that can be consumed by chemoautotrophs.
This is the most iffy, but can organisms harness liquid pressure to provide energy? I think piezoelectric materials could be used for this, but would energy be provided only as a conversion from kinetic energy (i.e. When something falls on it)?
Piezoelectric effects could be harnessed as a biological energy source, but only as a result of changes in pressure (something falling on it being an extreme example); constant pressure won't do anything for you.