Hi Nibbles,
I understand form your post, that you are unsure about your role and goals and I want to line out, that your goals and roles are free to you, also free to modify whenever you like.
This is unconvienient for a lot player. A often set goal is to enter the bigest bot in best equipment.
This will take a lot of NIC as well as several weeks until the EP are available.
Followed by the goal beeing able to have enough NIC, to replace this bot, when you loose it .
For the mix of combat and industrie in one agent you should be clear that a agent spending no EP in financial/mining/industrie but all in combat will be faster to have maxed combat skills. But after 3 months playing, the difference between lvl6 or level 8 is quite small.
A Miner will be faster having full benefit from mining, when not spending EP in combat skills, but again, after 3 month it is not that big deal, if someone gets 10 or 20% more ore per hour, as long as you have fun playing your agent.
Nevertheless a good number of Miner/Harvester have spend some EP in "Combat assistance" which is the logical role of a Miner/Harvester, while I know some people who spend EP in their Combat char for Indu skills like Repair/Recycling to be able to earn money without having an dedicated Indu Account.
The start specialisation leading to your start skills. You get a bit less than 10k (~1week of EP) for free with these start skills as well as several NIC you need to spend when purchasing the extension later. This is the only loss, you will have, when you detected later that you start in "wrong" corp. Regardless of combat/indu/commercial or if you detect too late, that you would prefer laser instead of EM or so.
A very profitable way for a miner to join combat with someone else would be to support a fighter with an mining/harvets bot fitted with energy transfer, remote repair and remote sensor. This support bot gives the fighter a decent boost as the fighter is then able to fit fully on dmg and neglect repair/energy.
If you check corp farm actions you see usually several of these support bots, as these boost farming significant. In my opinion these supporter are more important than a bot dealing dmg for succesfull farming on challenging spots.