It is a difficult task but this is how I have managed in the past. Take a glass Pasteur pipette, then heat this with a flame source and stretch the softened glass with forceps into a fine point. Break this off so you have a very small opening for isolating single diatoms.
I find it easiest to then attach a length of rubber tube to the pipette, then a rubber bulb to the other end of this, so I can have control of the pipette in one hand and suction in the other hand.
As for isolating individual cells it can be useful to use a needle or a seeker or similar to clear the majority of non-target material from the vicinity then try to draw up the cell of interest alone. I'd start at lower magnifications and then work up to higher ones. Holding breath after breathing out/avoiding coffee helps!
Yes it will be visible. Regarding objective, I think it's best to work with the lowest magnification you can, at 400x it'll be very challenging to control positioning and suction. It depends upon the size of your species of interest, perhaps try at x100 or x200 if that is possible?