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Andrew
Welcome to IIS!
Etepiece selection is an area that is very fraught for beginners and often for experienced observers as well. are you able to give us more details of the plossl eyepieces you already have? Often these scopes come with 2, often a 25mm and a 10mm. From experience, the 25mm is usually OK and the 10mm is not so. Reasons being the small eye relief and the narrow exit pupil. You may have already found this.
You suggest you are wanting high detail views on planets and are looking for power (magnification if you like) to do this. remember that if you increase the apparent size of the object you are viewing you are also increasing the size of any atmospheric blurring, and unfortunately there is nothing much that can be done about that (except moving to somewhere with steadier skies!!). The result is that high power images tend to get mushier as you increase the power so you need patience, especially with planets which are very strongly affected by seeing, to grab those fleeting moments of good seeing.
Getting to deep space stuff, you will find that for many objects there is a "sweet spot" as regards the amount of power you need to get the best out of it. This occurs as increasing power has 2 effects. Firstly it enlarges the apparent image that you are viewing which spreads the light over a larger area. As the total light you are collecting is still the same, that makes objects dimmer. But increasing power also darkens the sky background, so improving the contrast allowing the object to stand out a bit better. Objects with low surface brightness can benefit from higher power as a result.
All the above is a very simple explanation of some of the issues that you may have to think about, and I haven't touched on eye relief, exit pupils etc!
My usual advice to beginners is have a bit of a play with the eyepieces you have to get a better idea of how they work, and be prepared to realise that low to medium powers are where most of the fun is had. When I had my 12" dob, I did almost all my observing with a 24mm Panoptic that gave 63x and a 13mm T6 Nagler (same as yours!) that gave 115x. I did about 90% with the 24 and most of the rest with the 13. Occasionally I would pull out a 7mm Nagler but only rarely (and usually put it straight back in the case).
Anyway, hope the above helps!!
Malcolm
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