As I indicated, most apples have a SG in the range 1.045 to 1.055 so your Pink Ladies at 1.043 seem about right. The sugar level varies quite a bit with growing conditions. I have seen TA (total acidity) quoted for Pink Ladies in the range 3 - 5 grams per litre which is sort of "lowish middle of the road". Without high levels of acid, the sugar will dominate and so they will taste somewhat sweet but further fermentation will get rid of the sugar.
So, you should expect the Pink Ladies to pull the overall acid or tartness in your cider down, making it taste "sweeter". The existing yeast will have built up into quite a colony and should quite happily continue fermenting the Pink Lady sugars without any addition.
Although it is early in your cider journey, it might be worthwhile getting a Total Acidity Kit. These are relatively cheap and easy to use to measure the acid level in your cider. Be aware that they are usually "calibrated" to give the results in Tartaric Acid which is the dominant acid in wine. To get the results for Malic Acid (the dominant acid in Cider) you need to multiply the results by 0.89.
Andrew Lea (author of Craft Cider Making) has a good article about measuring cider TA with such kits on his web site. It looks a bit complex, but with a kit it is quite straightforward. Finding the article involves... go to cider.org.uk, it should take you to Wittenham Hill Cider Portal. Click on "press to learn more", look down the list on the LHS to find the article on measuring TA.
The reason for suggesting a TA kit is that you can measure what your current cider TA is, find the TA of your Pink Ladies, then start blending them into your cider until you get to, a target of say 5g/L which is a "normal" level. Alternatively, just start adding the Pink Ladies until the tartness reduces.
Without actually knowing the acid levels, I guess you have 5 litres of finished cider with a TA of maybe 8g/L. Adding 7 litres of Pink Lady at maybe TA of 4g/L would give you 40g of acid from the cider plus 28g of acid from the PLs for 68g of acid in 12 litres or about 5.5g/L which should be just about right.
Sorry about the long-winded explanation, but I hope it helps.
Cheers!