Resource allocation and traffic aggregation is becoming a significant part of the network deployment as it is expected that operators will offer advanced services through 5G systems. Scheduling procedures need to be fair but also to handle requirements set for diverse groups of users forming network slices. Heterogeneous networks will need to be controlled locally and the traffic could be split between the macro cell LTE nodes and small cell WiFi distributed access points (APs). In this work, we examine the performance of a novel optimal resource allocation algorithm while adding a new process aiming in achieving predefined throughput and delay performance for selected groups/slices of users. From the optimal resource allocation algorithm, we derive an algorithmic solution which can be applied to determine a number of network slices. The 3GPP-WLAN interworking setup evaluation shows that our proposal can be used for allocating resources to slices of users fulfilling throughput and delay requirements set by their subscription.