can I take some more of the fast acting insulin to compensate it or is that a no no?
Yes and no.
In these early days, you might be in honeymoon, so your pancreas might be producing insulin occasionally which might drag it down so a correction on top might drag it too low, although at 18 there's a lot of leeway.
Fast-acting operates over about 5 hrs. If it turns out that after 5 hrs you're back to where you started pre-meal then that indicates the amount was about right, but to avoid the spike some cautious experimentation with pre-bolusing might be useful - insulin takes about 20 mins to get distributed around the body before it starts working, so if you inject at the same time as a meal, the insulin will be playing catch up with the glucose, whereas injecting about 20 mins before lets the insulin deal head on with the glucose leading to a lower spike. The timing does vary between individuals, though, and will also depend on the gi of the food type though: pulses, for example take a long time to break down into glucose and be absorbed, so I'll rarely pre-bolus at all for a bean based meal, whereas if there's some toast involved, that'll break down pretty quickly, so 20 to 30 mins would suit that, for me at least. Be careful with this if you decide to try it - get it wrong and your bg can drop quite rapidly if the food is taking longer than expected to digest. Many people try it by adding 5 min increments with various food types to see what works for them.
The other issue is what bg level you started at before the meal. If you started at 5 or 6, up to 18 is a big bounce, and while pre-bolusing might have mitigated that a lot, there may also be questions about whether the dose amount was too little - seeing where it's at after 5 hrs will give clues on that. If, though, pre-meal was already 10 to 12, insulin has a much tougher job to do I can easily need twice the amount of insulin at higher levels - , so I would probably have been tempted to do a correction of a few units to bring it down to around 5 to 6 for eating - T1 sometimes requires patience!
The carb count will also play a part. Although we do have leeway in this, moderation also plays a big part. I'm not going go be hoovering up an 18 inch pizza and chips any time soon, but a half pizza and a side salad does the trick. Sometimes just reducing the portion size is enough to make it more manageable. I go to an Italian restaurant near me once a week or so - a panini and bowl of olives, say, 50g , or bit of lasagne, say 40g, does me fine, and about 80g of tortellini is good too, whereas I'm always a bit astonished by the people who are tucking into a bowl of spaghetti
and a plate of chips..
Once you get the basics like dose amount and timing right, and decide whether honeymoon is influencing things, then, yes, inter-meal corrections are a valuable tool. It's frowned on by nurses as it conflicts with DAFNE advice to avoid doing it, which advocates leaving corrections until meals, but, heck, we're grown adults and can make our own decisions about these things. Now that cgm is slowly becoming more common, many of us take the view that it makes perfect sense to correct if we see a line which left uncorrected would take us too high - why wait till you're too high when you can pin it while still in range? It is trickier to do with strips because it's not as easy to see the trend or keep track of insulin on board, but, yes, if I was at 18 I'd be tempted to do a correction between meals.
Here's an example of a correction I did. I'd pre-bolused 8u about 30 mins before lunch, put it down as 50g but it was at the cafe next door to my work so it was a bit of a guestimate. By 13:20 I could see it had been trending upwards for 20mins, you can't see it on the graph, but at the time there was a prediction showing and that and the trend and past experience were suggesting that I'd underestimated the carb count and it would carry on to above 8 so I fired in a 3u correction which pinned it nicely. It would normally only have been 1 ir 2u for that, but I quite like a biscuit or two in the afternoon, so thought I may as well do 3 to cover that too.