I'm going to make a few posts on 19nors in the coming week, one of which will cover this topic specifically.
Whether nandrolone causes mental sides is individual dependent. There are things you can do through supplementation to mitigate those sides to a degree as well. For example, nandrolone influences the body's demand for certain nutrients, particularly neurotransmitter precursors like tyrosine and tryptophan. For some people, taking more of these nutrients through supplements will mitigate nandrolone sides. With this example, the mental sides are just the result of an inadequate amount of neurotransmitter or dopamine precursors present.
Taking nandrolone in place of testosterone as a base is also a way that some people get around the mental sides. I find that my mood on nandrolone is highly dependent on how much testosterone I am taking with it. Ideally, I take nandrolone as the solo androgen with hcg for estrogen OR 5-10mg test prop or test ace daily or eod for estrogen. With this approach, I can take over a gram of nandrolone per week and feel perfectly fine. And I will have none of the dht and androgen sides of an equivalent amount of testosterone (like acne, oily skin, prostate enlargement, hair thinning).
The same is true of trenbolone. My tolerance for trenbolone is much higher in the complete absence of testosterone. This is because testosterone can be converted into neuroactive metabolites that may have stimulating and depressive effects. With testosterone present you also have aromatization to contend with, which can stimulate prolactin. Since both nandrolone and trenbolone have a higher binding affinity for the AR, there's a higher likelihood of unbound test floating through your system converting to other hormones causing chaos.
In your situation as a new user, I'd start with NPP at 15mg per day. Do that for 3 weeks, if you feel fine, bump it up to 25mg per week for another 3 weeks and repeat. I would not start with decanoate because if the depressive sides hit you hard, that's going to be a rough next 3-4 weeks and it does take some people out for a month like that, which can disrupt your work or whatever else it is that you're doing. With NPP, it's effectively out of your system in about a week and half after cessation of use.