Small systems might be trickier to start, that would depend on the details, but principally, you run into surface-to-volume effects. The smaller a system is, the more surface there is to create drag and take heat from the combustion stream, relative to the amount of heat generated. Volume is relative to the square of the surface, so combustion quantity is less and generates significantly less heat. At the same time a smaller system has less energy to flow, the surface drag increases. A smaller combustion volume will not be able to get as hot as a larger one because heat is bleeding out of it faster than it can build up, and you may not be able to get to complete combustion temperatures, leaving unburned gases in the exhaust which will condense to creosote.
You know what a regular red brick feels like, right? Hard firebricks feel distinctly heavier than that, and soft insulating firebrick feel considerably lighter. Insulating firebricks can be scratched with a fingernail, while hard bricks take work to cut with a masonry blade on a saw. Insulating firebrick would be ideal for a core, as long as you can be gentle with feeding wood. The feed tube probably wants to be lined with hard firebrick splits, or at least the top courses made of hard firebrick. You do need to mortar to seal all cracks so no cold air gets in where it is not supposed to. The fireclay mortar will work fine as long as you can keep the assembly from vibration or shocks and shifting. It is a sealant, not a cement or glue. A layer of straw-reinforced cob around the core will keep things in place even if you have IFB and additional insulation is not needed.
The 180 bends in a single 5" flue are definitely problematic. It is generally said that parallel flue paths are a bad idea, because they can never be identical and one will always end up with more flow than the other. However, short parallel flues may work fine in your case, as the friction in them will be much less than the friction in the connections before and after. Running three or even four 5' lengths of 5" duct will probably work fine, with a plenum at each end for gases to equalize. A few inches of cob over the assembly and you should get good results. You don't want more than a few inches of cover, as that (like 6" or so) would take hours to conduct heat to the outside and start warming your shop, and keep warming it far into the night after you leave.
A 6" firebrick J-tube can be tricky depending on your material. Insulating firebrick can easily be cut to whatever dimensions you like with a hacksaw, but hard firebricks are constrained by available sizes. If you cannot get a square cross section in the feed and burn tunnel, it is recommended that it is better to have it taller than wide. Standard firebrick dimensions will let you have 4 1/2", 5", or 7" height in various combinations of orientation. Using splits (4 1/2" x 9" x 1 1/4" thick) in the mix, you can get 5 3/4" high which would be close to perfect.