Hi MozSpearer, got your message. We are all happy to see youngsters who are keen on spearing post to the forum but are also a little wary about offering advice, lest it prove dangerous.
Glad to read that you will be with both your father & brother while in the water - are either/both experienced spearos/divers? Mozambique is a dream spearing destination for most of us - there are others on the forum that will be better able to offer advice specific to that location, but it has a good reputation for having excellent fishing.
If I recall correctly, the Tomahawk is an older green-handle model Rob Allen railgun - is that right? I believe they were/are good. I think there was some concerns about the strength of their early European-made mechs. when subjected to the rigors of long, double-banded guns but seem to recall that RA modified them accordingly ( - but please correct me if you know better). I guess the 1000 is a 100cm barrel - quite long for a 14-year old to load but relatively modest in length for Africa my most accounts - you are probably "in the ball park". Miles in Cape Town considered 110cm quite short & that is what he used on reefs. 110cm & 120cm seem to be popular sizes in S. Africa, 130/140+ and more for serious big game fishing. 100cm is likely close enough for a youngster starting out.
Don't spear fish that are too big for you to handle -- some big fish require large floats and/or double floats and sophisticated breakaway rigs to handle them. Don't shoot old, docile
Potato Bass. I would think sharks might be an issue in Africa - so think about what you will do with the fish you catch (which will likely release blood & fish oil into the water around you).
Don't push your breath-holding & make sure your dive buddies have a plan for what to do if anything goes wrong. Have you heard of "Shallow Water Blackout" ? Terry Maas published a good piece on the dangers of this [search this forum &/or Google for details}. Your breath-hold will usually, naturally starts out poor on a dive and improve throughout the day (and throughout the season).
Spearing is, I think, a "grown-up activity" - so try to adopt a calm, sensible, cautious, methodical approach to it. But enjoy it too! You may get yourself into positions where only you can sort it out. Try to think before hand about what you might need & how you might prepare - do you have a sharp safety knife/what happens if the boat breaks down/what if your father or brother is the one needing help/first aid kit & training... Have some back-up plans.
Flashers look fun, visibility here in the UK is rarely good enough to try them. Strung fish (which bleed & release fish oil) & gutted fish will often attract other fish in (in Africa, beware of sharks & other predators - you & family & friends will probably know more about that than us).
Good luck